1. Field of the Invention
The present invention relates to a wired spread spectrum communication device for communicating information signals that have been spread through spreading codes by means of transmission lines.
2. Description of the Prior Art
Conventional wired spread spectrum communication devices were arranged in that for the purpose of transmitting sync signals, sync bit strings comprised of, for instance, several tens of bits to several hundreds of bits were sent out in the form of so-called preamble signals prior to transmitting information signals of actual data. It was thus necessary to send such preamble signals each time synchronism was to be established with a counterpart side, which resulted in one cause of preventing improvements in transmission efficiency.
For solving such problems, the applicant (assignee) of the present invention has suggested “a wired spread spectrum communication device and a method for communication” as disclosed in Japanese Patent Unexamined Publication No. 2001-144653, wherein it is disclosed that the wired spread spectrum communication device is “a wired spread spectrum communication device for transmitting information signals by means of transmission lines, comprising a sync signal generator which generates sync signals that are synchronous with one of information signals, spreading codes for spreading information signals or spread spectrum signals after spread, and a transmitting means which sends out the sync signals to transmission lines together with spread spectrum signals”.
However, upon further studies that have been made by the inventors of the present invention, it has been found that the above-described wired spread spectrum communication device still exhibits the following drawbacks.
(1) It has been found that fluctuations in load connected to transmission lines for transmitting information signals or mixing of exogenous noise affect also properties of transmission lines so that cases may occur in which sync signals cannot be exactly extracted from received signals.
More particularly, when utilizing power lines such as power transmission lines or interior wirings as transmission lines, impedance properties of transmission lines fluctuate depending on load conditions of a demander side who receives supply of power or depending on supply conditions of a supplier side who supplies power, wherein such fluctuations cause attenuations in correlation peaks P to peaks P′ of waveform α as illustrated in FIG. 7(A). A drawback is accordingly presented in that establishment of synchronism becomes difficult.
Upon mixing of exogenous noise owing to electromagnetic waves or the like into transmission lines, peaks may appear at positions different from the original correlation peaks P that are shown as waveform β as illustrated in FIG. 7(A). Such peaks owing to exogenous noise also present the drawback that difficulties in establishing synchronism are found.
(2) When employing code division multiple access (hereinafter referred to as “CDMA”) through spread spectrum multiplex method, peaks exist at time positions that differ from those of the original correlation peaks P. A drawback is presented in that establishment of synchronism becomes more difficult owing to fluctuations in load or mixing of exogenous noise as already discussed in (1) above.
The present invention has been made in view of the above drawbacks, and an object of this invention is to provide a wired spread spectrum communication device, a method for communication thereof, and a wired spread spectrum communication system with which it is possible to easily establish synchronism.